The present exemplary embodiments relate to printing systems with raster output scanner (ROS) apparatus and to techniques for mitigating banding errors. Reprographic printing systems are used to create marked images on paper or other remarkable media, and improving the quality of the produced images is a continuing goal. Final image quality is affected by various sources of noise and errors in a reprographic system, leading to density variations in the marking material fused to the final print medium. In the reprographic process, a photoreceptor travels along a process direction, and images and text are formed as individual scan lines or groups of scan lines (sometimes referred to as a swath) in a raster scanning process in a cross-process direction, where the process direction motion is much slower than the raster scanning in the cross-process direction. Accordingly, the cross-process scanning direction is sometimes referred to as a “fast scan” direction, and the process direction is referred to as a “slow scan” direction.
Certain sources of reprographic system noise and errors caused periodic density variations in the process direction, which are sometimes referred to as “banding” errors. Periodic density variations may be characterized by the amplitude and phase of a fundamental frequency, as well as harmonics of this frequency. Various sources of banding exist in a marking (or print) engine. For example, raster output scanners employ rotating polygon mirror apparatus driven by a motor, known as a motor polygon assembly or MPA, with one or more light sources being scanned by rotation of the MPA such that scan lines are generated in the fast scan (cross-process) direction through reflection off a reflective facet of the rotating polygon mirror apparatus.
Differences in reflectivity, shape, profile, orientation, etc. in different reflective facets of the polygon lead to differences in image density (color intensity) in the final print out which are a function of which polygon facet was used to create a given scan line or swath of scan lines. As a result, the final print image may include bands of variations from the desired density that are periodic in the process direction. Other sources of banding errors include gears, pinions, and rollers in charging and development modules; jitter and wobble in imaging modules, as well as photoreceptors and associated drive trains. Banding usually manifests itself as periodic density variations in halftones in the process direction. The period of these defects is related to the once around frequency of the banding source. If not addressed, such periodic process direction density variations can render a reprographic printing system unacceptable, particularly where the banding errors are visually perceptible.
Banding can be addressed through reductions in the sources of such noise or errors and/or by compensation in various reprographic system components in order to counteract its affects, typically by injecting a known error that offsets the banding resulting from the sources of such periodic density variations. There are many various errors that produce banding at the 1× (and multiples) of the revolution frequency of the MPA (motor polygon assembly) in reprographic systems using a raster output scanner. In practice, it is difficult to completely eliminate the error sources that contribute to MPA harmonic banding, or even to reduce them enough to avoid perceptible periodic density variations. In addition, customer requirements are continually reducing the amount banding that is deemed to be acceptable. Consequently, banding compensation techniques have become an important tool in meeting reprographic system performance specifications. For instance, ROS exposure power can be varied in a controlled fashion to compensate for banding, and conventional banding compensation techniques include measurement of banding (including from multiple sources) and the use of that information to actuate some correction strategy on a scanline by scanline basis (including ROS exposure variation) to combat banding. However, conventional banding compensation approaches do not address cross-process (fast scan) direction density variation in banding, and instead average test prints in the cross-process direction to get a one-dimensional banding profile which is then used to derive the banding compensation independent of cross-process banding density variation information. Accordingly, there is a need for improved techniques for addressing banding errors in document processing devices and other systems using raster output scanners.